The installation of a brake modulator varies internationally according to different systems and standards, with both a direct connection to a compressed air reservoir and a connection via a pneumatic line being known. It is therefore advantageous to equip the brake modulator with two compressed air ports, both of which serve as compressed air inlets for relay valves. The redundant compressed air port may be closed, for example, by a plug, although connection of the two compressed air ports in parallel is also possible in principle.
Relay valves may be arranged in a longitudinal or axial direction of a common main housing, which may be configured as a light alloy die-casting. The relay valves may adopt their three valve positions—that is, the delivery position, the pressure-maintenance position and the vent position—independently of one another. A compact construction with a high air flow rate is therefore possible. The air supply may be effected, in particular, in a transverse direction (perpendicular to the axial direction) and centrally with respect to the compressed air inlets of the relay valves.
It is found, however, that such a construction, with the compressed air connection provided transversely, gives rise to a complex shaping of the main housing. The air delivery ducts extending from the compressed air port to a delivery valve seat are therefore generally configured as undercuts or similarly complex duct configurations in a body of the main housing.
Furthermore, the compressed air ports are typically provided from opposite sides and are axially offset from one another; such a pneumatic supply is, however, generally asymmetrical with respect to the relay valves, so that the relay valves may in some cases exhibit a non-uniform pressure build-up.